1. Field of the Invention
The present invention relates to a current summing type D/A (digital to analog) converter, an A/D (analog to digital) converter using the D/A converter, and a signal converter including the D/A converter or the A/D converter.
2. Description of the Related Art
There exists a semiconductor integrated circuit, which incorporates a large number of D/A (digital to analog) converters to perform signal processing. To achieve high accurate performance has been strongly required particularly to the D/A converter, and the resolution of the D/A converter has been improved by increasing the number of bits in a digital signal.
FIG. 10 is an example showing a signal converter, i.e., a laser output unit. In this signal converter, a current is provided to a laser diode (LD) to be emitted, and by adjusting a current value of the LD using the D/A converter, the laser diode is controlled so as to provide an optimum laser power.
In this signal converter, a digital-to-analog conversion is performed by a D/A converter 5 based on a current IREF outputted from a reference current source 1. More specifically, based on the current IREF, the D/A converter 5 outputs a current corresponding to a digital set point D9 to D0. In this signal converter, an ON/OFF control is also performed to the current outputted from the D/A converter using a current mirror CM11 provided with a switch SW101. Thus, a current provided to a laser diode (LD) 2 is ON/OFF controlled to thereby provide a laser output light to be switched. The current mirror CM11 is composed of transistors M101 and M102.
The laser light outputted from the laser diode 2 may be used as, for example, a recording signal for an optical disk such as DVD or CD. In this signal converter, the output current value is adjusted by changing the digital set point D9 to D0 of the D/A converter 5, allowing an optimum power output to be used as the recording signal for DVD and CD.
Apart of the laser light outputted therefrom is inputted to a photodiode (PD) 3. A voltage V101 is applied to the photo diode 3 shown in FIG. 10 by a DC power supply 101. A current generated by the laser light inputted to the photodiode 3 is current-voltage converted by a resistor R, and further analog-to-digital (A/D) converted by an A/D converter 6 through a DC voltage measurement circuit 4. The D/A converter 5 is controlled based on the digital data, thus making it possible to adjust the laser power.
FIGS. 11A and 11B are circuit configurations of a conventional D/A converter (10-bit configuration). FIG. 11A shows a schematic circuit diagram, while FIG. 11B shows a specific circuit diagram.
In FIGS. 11A and 11B, the reference current IREF inputted from a terminal IIN is outputted as a current from the current mirror CM12 composed of transistors Q212 and Q213. Weighted transistors Q201 through Q211 have a grounded-base configuration and are connected to a voltage source V201. The current IREF is shunt by the grounded-base transistors Q201 through Q211 and ladder connected resistors 102 of R-2R, and the resultant current is outputted to a terminal IOUT or a voltage source V202 through a set of switches 7 composed of switches SW201 through SW211. According to the aforementioned operation, the D/A converted current is outputted to the terminal IOUT.
The conventional D/A converter, however, has had two problems to be solved. One is a layout pattern, while the other is a change in characteristics due to manufacturing variability of the resistors.
The conventional D/A converter shown in FIGS. 11A and 11B requires the weighted grounded-base transistors Q201 through Q211. The attempt to increase the manufacturing accuracy for the transistors will necessarily result in an increase in size of the transistor Q211. The weighting may increase the transistor Q201, a tenth bit of the digital code, by 512 times of the transistor Q211 in size. As a result, the pattern layout configuration has been increased, while producing an irregular shape, so that the conventional D/A converter has had a problem that the optimization has been difficult.
Japanese Unexamined Patent Publication (Kokai) No. S62-214728 (Patent Application No. S61-56850) discloses an attempt to eliminate the weight for the transistors by adding a correction current to a base current of the transistor. The current variation produced by a change in resistance value of the resistorR caused by the manufacturing variability, however, could not be prevented. The change in characteristics caused by the variation in the resistance value of the resistor R has been a significant issue in the D/A converter, especially in the high-resolution D/A converter with a large bit number.
A connection relation among the switches SW201 through SW211 shown in FIG. 11B represents a current output state when a digital code “1000000000” is inputted. This corresponds to a next code of the digital code “0111111111”, so that the current value thereof must be equal to a current value of the digital code “0111111111” added to that of 1LSB. In other words, the current flowing through the transistor Q201 must be equal to the sum of the currents flowing through the transistors Q202 through Q211. In this case, even when the resistance value of the resistor R1 is deviated from a predetermined value thereof by 1/512=0.2%, that will lead the transistor Q201 corresponding to the upper bits to cause a current error by 1LSB.
Similarly, the current flowing through the transistor Q202 must be equal to the sum of the currents flowing through the transistors Q203 through Q211. When the variation by 1/256=0.4% of the resistance value of the resistor R2 is produced by the manufacturing variability, the current error will occurs in the output current by 1LSB.
An aspect of the current error affecting the characteristics in this case is shown in FIG. 12. Due to the variation in the resistance values of the resistors R1 and R2 shown in FIG. 11, the current error is produced in the upper 2 bits of the 10-bit D/A converter, and step heights (differential linearity error) are generated at three points in the output current characteristics as shown in FIG. 12.
In the signal converter for driving the laser diode shown in FIG. 10, the laser control current is feedback-controlled through a path from the D/A converter 5, the laser diode 2, the photodiode 3, the A/D converter 6, to the D/A converter 5. As a result, when the D/A converter has had the characteristics to generate the step height as shown in FIG. 12, there has been a problem that the current outputted to the laser diode 2 has been unstable due to the oscillation. The monotonically increasing characteristic as shown in FIG. 13 is therefore desired.
While Patent Application No. S61-56850 describes an example of a binary type D/A converter using the R-2R resistor, Japanese Unexamined Patent Publication (Kokai) No. S59-186416 (Patent Application No. S58-60881) describes an example of a current summing type D/A converter utilizing a combination of a segment type D/A converter and the binary type D/A converter using the R-2R resistor.
The errors produced in the current flowing through the D/A converter by the R-2R resistor and the current flowing through the segment type D/A converter will cause the differential linearity error during the D/A conversion. In Patent Application No. S58-60881, the current values flowing through the two D/A converters are corrected using a compensation circuit. The method described in Patent Application No. S58-60881, however, has not been able to prevent the differential linearity error due to the current error generated between the respective segments.
Japanese Unexamined Patent Publication (Kokai) No. S55-034536 (Patent Application No. S53-106675) describes a method in which the D/A conversion is achieved by connecting two current segment type D/A converters composed of current sources via the current mirror. Even by the method, however, the generation of the differential linearity error due to the error of the current mirror has not been able to be reduced.
While the device shown in FIG. 10 includes the D/A converter for converting the reference current, there is an apparatus that incorporates the D/A converter other than that. FIG. 14 shows a configuration of a comparison type A/D converter incorporating the D/A converter.
According to this comparison type A/D converter, a reference voltage VAREF supplied from a voltage source V501 is converted into a current I501 using a differential amplifier 8A and a resistor R501, and the converted current is transmitted to a D/A converter 9 via a current mirror CM13. Using the current I501 as a reference current, the D/A conversion is then performed in the D/A converter 9 to generate a current I502. The current I502 is supplied to a resistor R502 via a current mirror CM14. The comparison voltage VREF is thus generated. This comparison voltage VREF is inputted to a comparator 8B along with an analog voltage VAIN inputted from an external source, and those are compared in the comparator 8B to perform the A/D conversion. A control logic 10 is driven by a Clock to search a digital result using a binary search, and outputs the result to digital output terminals D9 to D0. If the D/A converter 9 being used then has a step height in input/output characteristics as shown in FIG. 12, since a plurality of digital values will correspond to a current value in the step height portions, the conversion error may occur. The D/A converter incorporated in the comparison type A/D converter will therefore be required to have the monotonically increasing characteristics as shown in FIG. 13 as its characteristic.
In a signal processing apparatus, which drives the laser diode with the current, there has been a problem that when the step height (differential linearity error) has been generated during the D/A conversion, the laser output has been unstable due to the oscillation in setting a laser intensity. In order to solve the problem, it is necessary to achieve the monotonically increasing characteristics not having the step height during the D/A conversion characteristics; namely, the differential linearity error is small.
Meanwhile, also in the comparison type A/D converter using the D/A converter, if there is the differential linearity error in the D/A conversion section, the error is generated during the digital conversion, so that a D/A converter having the small differential linearity error will be required.